When it comes to mowing with the tractor, the first is obviously safety. Never exceed the safety envelope for your machine on your property. More than likely, the second factor for general 'maintenance' mowing is comfort. Find a speed that you're comfortable with on the machine...without suspension, rough terrain can really do a number on the operator.
As far as RPM to run at, do not exceed the RPM marked on your gauge cluster for 540 PTO RPM. Usually there is an indicator somewhere between 2,000 and 3,000 RPM engine speed to indicate 540 PTO RPM. Consider this your MAX rpm to run (Do not exceed) when using the PTO. This is most likely NOT the 'rated RPM' of the tractor. IE: My L3800 has a 'rated' rpm of 2,700. The PTO operates at 540 RPM when the tractor is at an engine RPM of ~2,450, which is indicated by a yellow mark on the tachometer.
Now, you do not have to run at the 540 RPM if the job doesn't require it. I often run at 1,800 RPM, which is about 400 RPM PTO and it's more than adequate for maintenance mowing. If you find that your implement is not performing as expected, you probably could use more RPM. If your machine is having a difficult time with the load being produced, you certainly need more RPM.
There really isn't the perfect speed. It's up to the operator to determine based on the numerous factors involved.